The present invention relates to an imaging architecture for reproducing documents, and more particularly, to an imaging method and apparatus to process scanned images containing both halftoned and text/line regions.
In the reproduction or display of images from image data representing an original or scanned document one is faced with the limited resolution capabilities of the rendering system and the fact that many output devices are binary or require compression to binary for storage efficiency. Further complicating the reproduction or display of images is the reality that image output terminals must be able to process a variety of different image types including grayscale or continuous tones (contone), halftones of various frequencies, text/line art, etc. as well as image data comprising any combination of the above. Moreover, while an image processing system may be tailored so as to offset the limited resolution capabilities of the rendering apparatus, this tailoring is difficult due to the divergent processing needs required by different image types.
Optimizing a system for one common image type typically comes at the expense of degraded rendering of other image types. Consider, for example, printing a document having both contone pictorial data and text/line art on a binary printing system such as many xerographic or ink jet systems. Binary printing systems generally use a halftoning process to simulate continuous tone images. Conventional halftone screens employed by binary printers have a frequency approximately equal to 130-150 cpi dots. However, when rendering gray edge pixels, such as antialiased edge pixels common in text/line art, a very high frequency cell, ideally one having a frequency similar to the pixel resolution of the final output image, should be employed. Using a standard system halftone dot at the standard halftone frequency (e.g., approximately 130-150 cpi dots) to render antialiased pixels results in jagged edges often with objectionable halftone dots positioned along the edges of lines and characters. On the other hand, the use of a very high frequency screen over the entire image renders the antialiased pixel properly but introduces objectionable image artifacts in pictorial image areas and tends to sharpen the tonal curve and provoke print quality defects in the overall image.
A common goal in the development of printers and printing systems is improving image quality. High addressability imaging techniques have proven very successful in improving the image quality of printing systems. However, the divergent processing needs required by different images types are particularly evident in printing systems generating high addressability pixels. The use of high addressability rendering for both antialiased text/line art and pictorial contones has led to the development of printing systems having multiple rendering processors. Specifically, because available decomposers generally cannot perform high quality, high addressability rendering of text/line art, it is desirable to include dedicated hardware to perform high addressability rendering on antialiased text/line art in addition to a processor to render high addressability halftones. While such systems produce high quality output images, the need for multiple processors for different image types greatly increases the complexity and cost of the hardware and software modules required by the systems.
As with printing systems, improving image quality is a continuing concern when developing scanning devices and reproducing images from scanned data. To address the divergent processing needs of different images types when reproducing images from scanned data, scanning devices generally rely on automatic image segmentation techniques to identify different image types within image data and classify pixels accordingly. Based on the classification, the image data may then be processed according to the properties of that class of imagery. However, simply accurately identifying the image type does not guarantee image quality.
Consider the problem of scanning a halftone image. To accurately reproduce scanned halftone images, it is desirable to reproduce the screen of the printed halftone image. If the frequency of the scanned halftone is sufficiently low, below 130 cpi, many existing reproduction systems attempt to reproduce an image with its given halftone screen (i.e., no descreening) by employing simple thresholding, error diffusion or similar processing. While this halftone replication method works well for some low frequency screens, with middle frequency and higher frequency screens, it tends to introduce unwanted artifacts that degrade image quality. Thus, higher frequency halftones are typically low pass filtered (descreened) and then re-screened with a halftone that is suitable for the intended printer. While the above process accurately reproduces scanned images, it has some drawbacks. The descreening process typically introduces blur into the image. Furthermore, although passing scanned halftones using error diffusion can be used to accurately reproduce low frequency halftone screens without introducing serious artifacts, it does not ensure the rendered image has compact halftone dots resulting in images which are prone to noise and instability.
The following references may be found relevant to present detailed disclosure.
U.S. Pat. No. 5,274,472 to Williams discloses a method to convert gray level image data from image input terminals into binary data for high addressability image output terminals.
U.S. Pat. No. 5,485,289 to Curry discloses a printing system for rendering bitmapped image data on a photosensitive recording medium. The system includes a data source for supplying grayscale input image data and a scanning device for rendering grayscale output image data onto the recording medium.
U.S. Pat. No. 5,742,703 to Lin et al. discloses a method and apparatus for resolution enhancement of gray-scale input images including text and line art, and more particularly to a filtering method and image processing apparatus for enhancement of high contrast line edges found in grayscale images without requiring that the input image data include predetermined tag bits to identify region types.
In accordance with the present invention, there is provided a method of reproducing a document including halftoned regions therein scans the document to generate scanned image data comprising a digitized representation of said document. The scanned image data that corresponds to the halftoned regions is then rendered using a compact rendering process that includes positioning an observation window about a target pixel wherein the observation window includes a pixel neighboring the target pixel; analyzing the neighboring pixel; selecting a fill-order based upon a result of the analyzing step; and generating a high addressability pixel as a function of the selected fill-order.
In accordance with another aspect of the present invention, there is provided a system for reproducing a document, comprising a scanner for generating a digitized representation of the document, the digitized representation comprising a plurality of multi-bit pixels; a segmentation circuit coupled to receive the digitized representation from said scanner, the segmentation circuit classifying a target pixel into one of a plurality of image classes; and an image processing circuit, coupled to receive the digitized representation from the scanner and responsive to the segmentation circuit, wherein the image processing circuit includes a compact rendering module for rendering a target pixel classified as a halftone as high addressability pixels.